I have been riding with my ferrets (five of them) for about 3 years now.
When I first started taking them, I would put leashes on them, and carry
them in a backpack.  Now I wear a jacket, and put them in that (they still
wear leashes though).  They just go to sleep, except when I stop, and then
they stick their heads out and look around.  Occasionally one of them will
be insistant about getting out (usually to go potty), and I'll have to stop
and let them walk around for a while, but it's pretty rare.  When they try
to climb out of my jacket, I have plenty of time to stop and take care of
them, without much chance of falling.  That's what the leash is for.
 
I don't think that it's particularly dangerous for them.  As long as you're
a safe rider, and the ferrets stay in your jacket or bag, they should be
fine.  I wouldn't want to take them on a long trip (more than 100 miles or
so), shorter ones don't seem to be a problem.  I would recommend putting
then in your jacket instead of a backpack.  It's easier to keep an eye on
them.  I don't think a pet carrier is a good idea either - they'd probably
get tossed around too much.  I wouldn't take a bunch the first time you ride
with them, only take one or two, in case they decide they don't want to stay
put.  I'm sure it depends on the ferret too.  Mine are really laid back.  A
really hyper ferret might not behave well enough to ride with.
 
I'd say try it, and see how they react. Maybe you'll have new riding
partners :-)
 
--
Jay Monkman         The truth knocks on the door and you say "Go away, I'm
[log in to unmask]    looking for the truth," and so it goes away. Puzzling.
                     - from _Zen_and_the_Art_of_Motorcycle_Maintenance_
[Posted in FML issue 1462]